This poster describes how advanced surface micromachining
(SMM) technology is being used to develop prototype
cryogenic micromirror arrays for evaluation as an instrument
optical component for the NGST. When used as a spectrograph
reflective slit mask, these arrays can yield a factor of
~1000 reduction in mass and power over, traditional
motor-driven slit wheels used on HST instruments. The
advantage of micromirrors as a new approach to instrument
aperture control is particularly apparent when it is coupled
with new large format focal plane arrays to enable
multi-object spectroscopy. In this application, the
micromirror-enabled capability goes beyond mass and power
reduction to offer increased observing efficiency
(targets/hour). In the case of NGST, a factor of 100
improvement in efficiency relative to traditional instrument
designs has been estimated.

SMM devices use fabrication processes adapted from
integrated circuit manufacturing to build microscopic-sized
electromechanical devices from polycrystalline silicon.
Because these devices can be batch fabricated thousands or
even millions of devices can be fabricated on a single wafer
at costs several orders of magnitude less than
conventionally fabricated devices.

We describe how the SMM process is used to produce
micromirror arrays with 100-micron pixels in formats
scalable to 2048 x 2048 assemblies. Each pixel consists of a
mirror, which is held in position by an appropriately
designed suspension, and electrodes that are used to
electrostatically tilt the mirror pixel to the desired
orientation. The Sandia National Laboratories SMM process
uses four independent layers of polycrystalline silicon to
produce the appropriate structures. A special
chemical-mechanical polishing process is used to produce a
very flat top mirror surface. The design of a typical
mirror, and how the process is used to create the desired
structures is described and illustrated.

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